The paucity of activated T cells infiltrating established tumors in immunocompetent hosts helps to explain the inability of hosts to dispose of tumors. Experiments in animal models as well as clinical studies indicate that the immune system can recognize and kill individual tumor cells, but a host cannot generally eradicate established solid tumors. There may be several explanations for the failure of the host to respond effectively to established tumors: 1) lack of early T cell priming due to poor direct or indirect presentation in lymphoid tissues because of an inadequate number of tumor cells (especially those of non-hemopoietic origin) migrating to the lymphoid tissue; 2) inadequate numbers of immune cells migrating to tumor sites due to biological barriers around tumor tissues; 3) exhausted or short-lived activated antigen-specific T cells that fail to combat tumor growth due to limited repertoires; 4) unresponsiveness or ignorance of T cells to tumors; 5) an inhibitory microenvironment or lack of stimulation inside tumors to activate the immune system.
Clinically, increase of infiltration of T cells to the tumor site is closely associated with better prognosis. Previous studies have shown that preventive vaccinations were effective in inducing the rejection of inoculated tumor cells. After tumor growth has been established, however, the therapeutic vaccinations usually fail to reject tumors. Surgical debulk of tumor does not boost the immune response to tumors. Furthermore, it was reported that even the expression of a strong antigen on tumor cells was insufficient in promoting the rejection of an established tumor, despite the presence of excessive numbers of antigen-specific T cells in the lymphoid tissues. Lack of T cells priming and/or infiltrating an established tumor is one of the major obstacles for either natural or therapeutic approaches against antigenic cancers. In addition, insufficient expression of costimulatory molecules inside tumor tissues may fail to activate infiltrating T cells and result in the anergy of tumor-reactive T cells.
The lack of early T cell priming is possibly attributed to only a few tumor cells that migrated from solid tissue to lymphoid tissues for direct presentation. Genetic analysis using bone marrow chimeras has revealed two modes of antigen presentation for priming MHC-I-restricted CD8+ T cells. Direct-priming is mediated by the engagement of T cells with the cells that synthesize the protein with antigenic epitopes, whereas cross-priming is mediated by the host antigen-presenting cells that take up antigens synthesized by other cells. The mechanisms for priming tumor-specific T cells has been vigorously debated and so far remains inconclusive. Understanding how and where tumor antigens are presented to T cells would help find a therapeutic action against tumors.
LIGHT (homologous to lymphotoxin, exhibits inducible expression, and competes with HSV glycoprotein D for herpes virus entry mediator, a receptor expressed by T lymphocytes) is a recently identified type II transmembrane glycoprotein of the TNF ligand superfamily. LIGHT (TNFSF14) is a tumor-necrosis factor (TNF) family member that interacts with Lymphotoxin β receptor (LTβR) and herpes virus entry mediator (HVEM) mainly expressed on stromal cells and T cells, respectively. LTβR signaling is required for the formation of organized lymphoid structures, which can be attributed, at least in part, to its ability to induce the expression of chemokines and adhesion molecules that attract naïve T cells and dendritic cells (DC) in lymphoid organs. Stimulation of LTβR on stromal cells by LIGHT in vivo leads to the expression of CCL21, which attracts naïve T cells in the T cell area of the spleen in the absence of LTαβ, another ligand for LTβR. These results demonstrate that LIGHT is able to interact with LTβR to regulate CCL21 chemokine expression. In addition, LIGHT also exhibits a potent, CD28-independent co-stimulatory activity for T cell priming and expansion leading to enhanced T cell immunity against tumors and/or increased autoimmunity. Signaling via LTβR is required for the formation of organized lymphoid tissues. Lymphotoxin β receptor (LTβR) plays an important role in the formation of lymphoid structures. LTβR is activated by two members of the TNF family, membrane lymphotoxin αβ and LIGHT (FIG. 1). LTβR plays pivotal roles in the formation of LNs and the distinct organization of T, B zones in secondary lymphoid organs. Signaling via LTβR regulates the expression of chemokines and adhesion molecules within secondary lymphoid organs. Chemokines and adhesion molecules control the migration and positioning of DCs and lymphocytes in the spleen. Over-expression of soluble LT or TNF in non-lymphoid tissues was sufficient to promote functional lymphoid neogenesis.
LIGHT plays a unique role in T cell activation and the formation of lymphoid tissue. LIGHT is a ligand for LTβR and herpes virus entry mediator (HVEM). LIGHT is predominantly expressed on lymphoid tissues. Interactions between LIGHT and LTβR restore lymphoid structures in the spleen of LTα−/− mice. In addition, upregulation of LIGHT causes T cell activation and migration into non-lymphoid tissues and forms lymphoid-like structures. Conversely, LIGHT−/− mice showed impaired T cell activation and delayed cardiac rejection. Therefore, LIGHT is a potent costimulatory molecule that also promotes the formation of lymphoid tissues to enhance local immune responses. Lack of efficient priming of naïve T cells in draining lymphoid tissues and the inability to expand tumor-specific T cells within tumors prevent the eradication of cancer.
Micrometastases (small aggregates of cancer cells visible microscopically) can become established at a very early stage in the development of heterogeneous primary tumors and seed distal tissue sites prior to their clinical detection. For example, the detectable metastasis in breast cancer can be observed when the primary tumor size is very small. Therefore, at the time of diagnosis many cancer patients already have microscopic metastasis, an observation that has led to the development of post-surgical adjuvant therapy for patients with solid tumors. Despite these advances, success has been limited, and optimal treatment of metastatic disease continues to pose a significant challenge in cancer therapy.
A variety of human and murine cancers have been proven to be antigenic and able to be recognized by T cells. Tumor-reactive T cells could theoretically seek out and destroy tumor antigen-positive cancer cells and spare the surrounding healthy tissues. However, the naturally existing T cell responses against malignancies in human are often not sufficient to cause regression of the tumors, primary or metastases. It has been recently reported that sporadic spontaneous but immunogenic tumors avoid destruction by inducing T cell tolerance but that activation of tumor antigen-specific T cells may completely prevent the development of spontaneous tumors. Thus, breaking tolerance and generating such T cells capable of rejecting tumors around the time of treatment of the primary tumor could represent a potential approach to clearing metastatic tumor cells. As antigen-lost variants can escape under immunological pressure, immunotherapy should be applicable independent of knowledge of specific tumor antigens.
From an immunological perspective, present clinical strategies hinder the immune defense against malignancies and further diminish the effectiveness of immunotherapy. Although removal of a tumor may reverse tumor-induced immune suppression, surgical excision of the primary tumor before immunotherapy also removes the major source of antigen, which may lead to a reduction of the activation of CTLs since the efficiency of priming is correlated with the tumor antigen load. In addition, currently utilized adjuvant treatment with chemotherapy and radiation therapy that is meant to kill residual tumor cells may in fact impair anti-tumor immune responses by destroying or inhibiting T cells.
A challenge in developing an effective immunotherapy is to devise an approach to increase the number or enhance the function of circulating tumor-specific T cells that may detect and destroy microscopic metastatic cells before they become clinically meaningful.
In the present disclosure, inducing immune responses in tumor tissues via mutant LIGHT prior to surgery generated sufficient primed antigen-specific effector T cells to exit the tumor and eradicate metastasis. Delivery of mutant LIGHT, using a recombinant adenovirus into to the primary tumor of 4T1 mammary carcinoma bearing mice also mediated prevention and eradication of spontaneous metastasis.